Somalia's new leader is appealing for peace and unity after four insurgent factions, opposed to his leadership of an anticipated unity government for Somalia, agreed to merge and continue fighting.

President Sheik Sharif Sheik Ahmed responded late Thursday to one of the earliest challenges to his authority. He asked the newly-created opposition group to lay down their arms and join the government.

"We are requesting our brothers to work with us in restoring peace and unity," the Somali leader said. Sheik Sharif says he is willing to open a dialogue with all opponents and would welcome their participation in government.

The new, anti-government Islamic Party is composed of four factions - the hard-line Asmara wing of the Alliance for the Re-liberation of Somalia led by Hassan Dahir Aweys; Harakat Ras Kamboni, a southern Somali Islamist group affiliated with radical military leader Hassan Turki; the Islamic Front, an insurgent group formed in 2007 to oppose Ethiopian troop presence in Somalia; and a little-known, clan-based group called Anole, which is said to have been founded just weeks ago.

In a statement announcing the group's formation on Wednesday, the spokesman of the Islamic Party Hassan Mahdi criticized Sheik Sharif's participation in U.N.-sponsored talks in Djibouti. The talks paved the way for the withdrawal of Ethiopian troops from Somalia last month and allowed Sheik Sharif and his opposition party to join the transitional federal government.

Mahdi said the Islamic Party was formed to counter moves by Sheik Sharif to turn Somalia into a secular, pro-western state.

He says Sheik Sharif betrayed Islam when he went to Djibouti to negotiate with the government installed by Ethiopia.

The transitional federal government took power in late 2006 after the Ethiopian military overthrew the Islamic Courts Union. The intervention ignited a two-year Islamist-led insurgency.

As one of the leaders of the Islamic Courts Union, Sheik Sharif initially fled to Asmara, Eritrea to form the opposition Alliance for the Re-liberation of Somalia with Hassan Dahir Aweys. But the alliance split after Sheik Sharif agreed to begin peace talks with the transitional federal government last June. Following the split, Sheik Sharif re-located his group to Djibouti.

The strongest of the Somali insurgent groups, the militant al-Shabab, is considered the biggest threat to Sheik Sharif's attempts to form a new unity government. Al-Shabab, whose leaders have condemned the moderate Islamist for reaching out to the West, control many major cities in southern and central Somalia and has vowed to continue fighting until all of Somalia is under strict Islamic law.

Last Saturday's parliamentary elections that brought Sheik Sharif to power had to be held in Djibouti because the parliament's seat in Baidoa, Somalia was taken over by al-Shabab after the Ethiopians withdrew.

Mahdi says al-Shabab is not a member of the Islamic Party. But he says the al-Qaida-linked group, labeled as a terrorist organization by the United States, may be persuaded to join later.

A Somali social activist, who requested not be identified because of security reasons, says he believes the Islamic Party will not find much popular support in Somalia. But he says the group could get backing from Ethiopia's main rival in the region, Eritrea, which has long been accused of supporting Somali insurgents in a proxy war against Ethiopia.

"I think the main source they are relying on for support [is] from Eritrea - financially, politically, and militarily. If they achieve [this], it will inflict damage on the new government," the activist says. "But if the international community fully supports this new government, I think it will be able to come up with a stabilization process in Somalia, which is highly needed." That stabilization process will also have to include efforts to be recognized by the semi-autonomous Somali region of Puntland and the breakaway republic of Somaliland. Both have yet to give their approval of Sheik Sharif's presidency.